The economy of RNA and DNA

Living systems look awfully complex. Why there are both DNA and RNA? Why Thymine is replaced with Uracil in RNA? Isn’t it much simpler with one set of molecules? A little bit economics will greatly simplify our understanding of living systems.

Essentially, we are asking why there are college graduates and high school graduates in a society. The short answer is that it is cheaper to have high school graduates. DNA are college graduates. RNA are high school graduates.

The relation between DNA and RNA is difficult  to understand because we often think from the perspective of DNA, the more dominant molecules. If we think from the perspective of RNA, the less well known molecules, the economic logic becomes very simple.

DNA is Deoxy RNA. It takes one more step to transfer RNA into DNA. This makes DNA molecules chemically more stable than RNA molecules. DNA molecules are used in functions requiring long term stability, such as genes. Economically, DNA is more expensive to make than RNA. For short term works, such as copying and sending messages, RNA molecules can fulfill the duties cheaper. That is why both DNA and RNA are useful and used in living systems.

We are often puzzled why Thymine is replaced by Uracil in RNA. It is easier to understand why Uracil is replaced by Thymine in DNA. We know that RNA is cheaper than DNA. For short term work, simple and cheap Uracil is suffice. Being the simplest, Uracil also generates Cytosine, another base in DNA and RNA. Cytosine could degenerate into Uracil. When this happens, we can’t distinguish Uracil from Cytosine. Uracil is not suitable for DNA, which requires high level of fidelity in information storage and transmission.

To resolve this problem, we have to turn Uracil into a more distinguishable form.  That is Thymine, which is made from Uracil with an added methyl group. With an added cost, Uracil is turned into Thymine, which is more distinguishable and hence generates less mistakes. In RNA, low cost Uracil is used. In DNA, high cost and more distinguishable Thymine is used.

In biology, it is simpler to understand living systems from the perspective of humbler RNA. Yet we tend to think from the perspective of more prominent DNA. Similarly, in human societies, it is simpler to understand social systems from the perspective of the meek. Yet we tend to think from the perspective of more prominent people.